PRML (Partial-Response signaling with Maximum-Likelihood sequence detection) is a technique for processing data read from a magnetic support (such as a hard disk or a magnetic tape), that provides for the use and sampling of only a part of the input signal's frequency band and the search for the maximum likelihood of correspondence of the levels of the output signal with the samples obtained from the input signal.
In an existing PRML reading/writing channel, the analog signal from the magnetic head is processed and reconstructed digitally through an apparatus that carries out a first analog equalization by means of a variable-gain input amplifier and a continuous low-pass filter. At the filter output, with a suitable selection of the cut-off frequency, the high-frequency accentuation (boost), and other parameters such as group delay, it is possible to obtain a signal with a minimum InterSymbol Interference (ISI). The output of the analog filter is then sampled through an analog-to-digital converter and subsequently processed through a transversal digital filter (called Finite Impulse Response (FIR)) that further reduces the ISI. The FIR filter output signal may be processed by another filter (Pole Tip Filter) that eliminates the small undesired peaks produced when the magnetic support enters or leaves the active area of a thin-film reading head. The output of the Pole Tip Filter (or the FIR filter if the Pole Tip Filter is not used) is processed by a Viterbi detector i.e., a maximum-likelihood sequence detector operating with the Viterbi algorithm (which is described in G. D. Forney, Jr., "The Viterbi Algorithm", Proc. IEEE, vol. 61, No. 3, pages 268-278, March 1973) which converts a sequence with many bits into a sequence with just one bit on the basis of a maximum likelihood concept). The output of the Viterbi detector is processed by a decoder capable of converting the Run Length Limited (RLL) code commonly utilized for writing data on a magnetic support into the Non-Return to Zero (NRZ) code commonly utilized for allowing a user to use the read signal.
There are also two feed-back loops. The first, which contains a unit for controlling the gain of the input amplifier, allows the stabilization of the input signal's amplitude. The second, which contains a unit for controlling the timing, allows the timing signal of the analog-to-digital converter to be constructed from the data read from the magnetic support.
When an apparatus as described above is used for reading data stored on a hard disk, it is typically provided with a device that processes special signals prerecorded on the disk by the manufacturer (normally called "servo signals") to obtain appropriate signals for operating the actuator of the head's support arm so as to control the position of the reading head.
In order to attain high track densities with a low likelihood of off-track operation, the existing technique provides for the extraction of the integral of the servo signals' module (typically 4 bursts at the PRML channel's maximum response frequency) in a predetermined timing window, and for the passage of the analog value of the integrated voltages to an external, high-resolution (10 bit) analog-to-digital converter. The output of the analog-to-digital converter is processed by a digital signal processor that controls the actuator of the head's support arm.
The operation of integration is necessary to reduce the effects of the noise introduced in the reading process, a noise that increases as the distance between adjacent tracks decreases.
An existing device for processing servo signals using this technique in a PRML reading apparatus as described above comprises in particular an analog demodulator formed by a full-wave rectifier, which receives and rectifies the analog signal at the output of the low-pass filter, an integrator, and an S/H buffer with an analog multiplexer, which together, store the detected signals and transmit them in time succession to the external analog-to-digital converter.
In European Patent Application No. 94830236.9 filed on May 23, 1994 by the applicant of the present application, there is described a new and advantageous parallel architecture PRML reading apparatus, wherein the transversal filter is of the analog continuous-time type and is located at the output of the low-pass analog filter. Between the transversal filter and the RLL-NRZ decoder there are two parallel processing channels. Each channel comprises an analog-to-digital converter and a Viterbi detector arranged in series and operates according to sampling sequences that alternate with one another. EPA 94830236.9 is incorporated by reference herein for all purposes.